Some plerion-type SNRs are surrounded by shells of emission resulting from the forward shock interacting with the surrounding medium. Such remnants are often called “composite type." One of the typical composite type SNRs is
G21.5\(-\) 0.9 [1]}.
Figure REF  is the X-ray image of G21.5\(-\) 0.9. One can see both the central PWN and the outer shell.
